The present invention relates to a decision feedback equalizer and, more particularly, to a decision feedback equalizer to be used at a receiving station in a digital radio communication system which used multilevel quadrature amplitude modulation or multiphase phase modulation.
In a modern digital radio communication system, a receiving station uses a transversal equalizer with a transversal filter in order to overcome the degradation of a channel ascribable to frequency-selective fading which occurs on a transmission path. A decision feedback equalizer is even superior to such an equalizer regarding the equalizing characteristic. A decision feedback equalizer is disclosed by D. A. George et al in "An Adaptive Decision Feedback Equalizer", IEEE TRANSACTIONS ON COMMUNICATION TECHNOLOGY, Vol. COM-19, No. 3 June 1971, pp. 281-293.
When deep fading, e.g., two wave fading occurs, the level of a baseband signal being outputted by an demodulator sometimes becomes higher than a predetermined level. Then, the frequency characteristic inside the band noticeably varies and, in the worst case, exceeds the predetermined input range of an analog-to-digital (A/D) converter which follows the demodulator, preventing a received signal from being adequately equalized. To eliminate this problem, i.e., to prevent the A/D converter input from exceeding the predetermined level despite deep fading, it has been customary to interpose between the demodulator and the A/D converter a gain control circuit which compresses the amplitude of the demodulated signal to 1/K (k.gtoreq.1) and applies the amplitude-compressed signal to the A/D converter. However, such a conventional implementation causes the equalizing ability to lower in the event of two wave fading in which a principal wave and a delayed wave have substantially the same level.